Re: [PATCH net] ipv6: sr: restore network header before routing and forwarding

[Ido Schimmel <idosch@nvidia.com>]

On Fri, Aug 28, 2026 at 02:17:27PM +0000, Eric Dumazet wrote:
> ipv6_srh_rcv() runs with skb->data at the Segment Routing Header (SRH)
> while skb_network_header() points at the IPv6 header.
> 
> When segments_left > 0, ipv6_srh_rcv() previously restored the skb->data
> position by pushing sizeof(struct ipv6hdr), assuming the SRH immediately
> followed the fixed IPv6 header. If another extension header (such as a
> Hop-by-Hop options header) precedes the SRH, skb_network_offset()
> remained negative.
> 
> This led to two problems:
> 1. During ip6_route_input(), fib6_rules_early_flow_dissect() invokes
>    __skb_flow_dissect() which passes the negative skb_network_offset()
>    to flow dissection, breaking BPF and C flow dissector logic.
> 2. If forwarded via ip6_forward() or redirected via act_mirred, downstream
>    handlers (like sch_fragment() or neighbour output) pass the negative
>    offset as an unsigned length, triggering OOB memcpy or buffer overflows.
> 
> Fix this by pushing -skb_network_offset(skb) before routing, ensuring
> skb_network_offset(skb) is 0 for route lookup / flow dissection as well as
> downstream forwarding. On the loopback path, pull skb_transport_offset(skb)
> to restore skb->data to the SRH before looping back.
> 
> Fixes: 1ababeba4a21 ("ipv6: implement dataplane support for rthdr type 4 (Segment Routing Header)")
> Reported-by: TencentOS Corvus AI <corvus@tencent.com>
> Reported-by: Jun Yang <junvyyang@tencent.com>
> Reported-by: Fourie Zhang <fouriezhang@tencent.com>
> Closes: https://lore.kernel.org/netdev/20260817104128.22681-1-juny24602@gmail.com/
> Closes: https://lore.kernel.org/netdev/20260827092345.2301937-1-fouriezhang@tencent.com/
> Signed-off-by: Eric Dumazet <edumazet@google.com>

Reviewed-by: Ido Schimmel <idosch@nvidia.com>